The present invention relates to silicone compositions suitable for forming pressure sensitive adhesive compositions. More particularly, the present invention relates to low viscosity, addition-curable silicone compositions which have a high solids content and which cure to form pressure sensitive adhesive compositions having excellent tack and peel adhesion properties.
The term "pressure sensitive adhesive" (PSA) as used herein refers to adhesives that can be adhered to a surface and yet can be stripped from the surface without transferring more than trace quantities of adhesive to the surface, and can be readhered to the same or another surface because the adhesive retains some or all of its tack and adhesive strength.
Silicone pressure sensitive adhesives have an excellent adhesive strength, tack, and cohesive strength, which are the properties required of a pressure sensitive adhesive. In addition, they also have the heat resistance, cold resistance, electrical properties, and the like, characteristic of silicones, and so are widely used for electrical-insulating tape, which must be highly reliable, and for various pressure-sensitive products which must be resistant to hot and cold.
A drawback associated with the use of silicone pressure sensitive adhesives is the use, handling, and emission of flammable and volatile organic compounds, e.g., organic solvents, in the preparation of the pressure sensitive adhesives from compositions containing high levels of organic solvent. Solvent is generally used for the purpose of reducing the viscosity of the composition to a level which renders the curable composition processable. It is desirable, therefore, to provide solventless or high solids containing (i.e., low solvent-containing) polydiorganosiloxane compositions for use in the preparation of pressure sensitive adhesives.
Another drawback associated with conventional pressure sensitive adhesives is the requirement for a high temperature, e.g., 165.degree. C., to effectively cure the pressure sensitive adhesive using a peroxide catalyst. Such pressure sensitive adhesives cannot be used on substrates sensitive to high temperatures, for example, polyolefin-backed substrates, due to their temperature incompatibility.
Addition-curable silicone compositions capable of curing to form pressure sensitive adhesive compositions are known in the art.
European Patent Application No. 0355991 (Boardman) is directed to a pressure sensitive adhesive composition having a high solids content, i.e., typically in excess of 95% by weight, preferably in excess of 98% by weight, comprising (a) a benzene soluble resinous copolymer consisting of R.sup.1 R.sup.11 R.sup.111 SiO.sub.1/2 units and SiO.sub.4/2 units and containing silicon-bonded hydroxyl radicals ranging from 1 to 4 percent by weight of the copolymer, (b) a diorganoalkenylsiloxy endblocked polydiorganosiloxane, (c) a diorganohydrogensiloxy endblocked polydiorganosiloxane, (d) a crosslinking agent, and (e) a hydrosilation catalyst. The crosslinking agent used in Boardman is selected from low and high molecular weight polyorganosiloxanes having the general formulae ##STR1## wherein each R.sup.6 represents a monovalent saturated hydrocarbyl group, n and m are each numbers from 0 to about 1000, p represents 3 or 4, Z is a vinyl radial or hydrogen, and M is a silyl group selected from R.sup.a R.sup.b R.sup.c Si and R.sup.d R.sup.e ZSi, wherein R.sup.a, R.sup.b, R.sup.c, R.sup.d, and R.sup.e are each monovalent saturated hydrocarbyl groups. The ratio of the total number of alkenyl groups present in (d) and (b) to the total number of silicon-bonded hydrogen atoms present in (d) and (c) ranges from 0.8 to 1.2, i.e., the ratio of the total number of silicon-bonded hydrogen atoms to the total number of alkenyl groups ranges from 0.83-1.25:1. The Examples presented in Boardman show that the pressure sensitive adhesives prepared therein had low to moderate tack properties. The Boardman patent does not teach that useful pressure sensitive adhesive properties (especially tack) can be obtained at SiH/SiVinyl ratios of greater than 1.25:1. It would be desirable to provide solventless or high solids-containing pressure sensitive adhesives having higher tack properties as well as high peel adhesion. Furthermore, the Boardman patent does not disclose the connection between the combined hydride equivalent weight of the hydride-containing polydiorganosiloxane crosslinker and the hydride-terminated polydiorganosiloxane fluid and the ultimate PSA properties.
U.S. Pat. No. 3,983,298 (Hahn et al.) is directed to a composition suitable for use as a pressure sensitive adhesive and obtained by mixing components consisting essentially of (a) 50-60 parts by weight of a solid, benzene-soluble resin copolymer consisting essentially of R.sub.3 SiO.sub.1/2 units and SiO.sub.4/2 units, (b) 40-50 parts by weight of a vinyl-terminated polydiorganosiloxane having a viscosity of from 20,000 to 100,000 centipoise at 25.degree. C., (c) a hydrogen-containing organopolysiloxane in an amount sufficient to provide from 1.0 to 20.0 silicon-bonded hydrogen atoms of every olefinically unsaturated radical in the total of (a) plus (b), and (d) a platinum-containing catalyst. It is pointed out in Hahn et al. that compositions of the prior art based on mixtures of a benzene soluble resin copolymer containing R.sub.3 SiO.sub.1/2 units and SiO.sub.4/2 units (hereinafter somtimes referred to as "MQ resin") and low viscosity silicones do not form pressure sensitive adhesive compositions.
U.S. Pat. No. 4,774,297 (Murakami et al.) teaches the preparation of pressure sensitive adhesives using vinyl functional polysiloxanes having even higher viscosity than those used in the Hahn et al. patent discussed above. The patent to Murakami et al. is directed to a composition suitable for forming a pressure sensitive adhesive having excellent tack and adhesive strength, comprising (A) 30-70 parts by weight of a vinyl-terminated polydiorganosiloxane having a viscosity of at least 500,000 centipoise at 25.degree. C., (B) 70-30 parts by weight of an organopolysiloxane containing R.sub.3 SiO.sub.1/2 units and SiO.sub.4/2 units, (C) an organohydrogensiloxane in an amount sufficient to provide 1-20 silicon-bonded hydrogen atoms per alkenyl group, (D) a platinum-containing catalyst, and (E) from 25-400 parts by weight of an organic solvent. In order to obtain a satisfactory product, Murakami et al. teach that it is essential that the vinyl polymer have a viscosity of at least 500,000 centipoise and preferably at least one million centipoise at 25.degree. C.
European Patent Application No. 0269454 (Murakami et al.) discloses a composition suitable for forming a pressure sensitive adhesive having excellent tack and adhesive strength and comprising an alkenyl group-containing silicone polymer, a tackifying silicone resin, an organohydrogensiloxane, and a platinum-containing catalyst. According to the Murakami et al. (Murakami) reference, there is no specific restriction on the molecular weight of the alkenyl group-containing silicone polymer as long as no adverse effect accruses with respect to the workability of the pressure sensitive adhesive composition. If the pressure sensitive adhesive composition is solventless, the viscosity of the alkenyl group-containing silicone polymer is no more than 100,000 centipoise at 25.degree. C. In a solvent-containing composition, the alkenyl group-containing silicone polymer should have a viscosity of at least one million centipoise at 25.degree. C. The organohydrogenpolysiloxane should be present in an amount sufficient to provide 1 to 40 silicon-bonded hydrogen atoms per alkenyl group in the composition. The patent to Murakami et al. does not disclose a pressure sensitive adhesive composition using low viscosity vinyl-functional silicones wherein the composition has excellent peel adhesion and high tack properties.
U.S. Pat. No. 4,988,779 (Medford et al.) disclose a composition suitable for forming a pressure sensitive adhesive, the composition having a solvent content of no more than 5-10% by weight and comprising from 30 to 50 parts of a vinyl endblocked polydiorganosiloxane fluid having a viscosity of from 500 to 10,000 centipoise at 25.degree. C., from 50 to 70 parts of a benzene soluble resin copolymer containing R.sub.3 SiO.sub.1/2 units and SiO.sub.4/2 units, an organopoly-siloxane having silicon bonded hydrogen atoms, and a platinum catalyst. The hydrogen-containing organopolysiloxane is present in an amount sufficient to provide from 1.0 to 30.0 silicon bonded hydrogen atoms for every olefinically unsaturated radical in the composition. Although the composition disclosed in the Medford et al. patent uses a low viscosity vinyl-functional silicone, it is continually desirable to provide pressure sensitive adhesive compositions which use even lower viscosity vinyl-functional silicones.
It is continually desirable to provide solventless or high solids-containing, addition-curable polydiorganosiloxane compositions which cure to yield silicone pressure sensitive adhesives having high tack and excellent peel adhesive strength. It is further desirable to provide solventless or high solids-containing, addition-curable silicone pressure sensitive adhesive compositions which can be cured at relatively low temperatures to yield silicone pressure sensitive adhesives having high tack and excellent peel adhesive strength.
In the present invention, it was found that an addition-curable silicone composition containing a vinyl-terminated polydiorganosiloxane having a viscosity of less than 500 centipoise at 25.degree. C. can be cured at relatively low temperature to form pressure sensitive adhesives with both high tack and high peel adhesion, if a particular type of external hydride crosslinking agent is used, if the combined average hydride equivalent weight of the hydride crosslinking agent and the hydride silicone fluid in the addition-curable composition are within a certain range, and if the ratio of silicon-bonded hydrogen groups to silicon-bonded alkenyl groups is within the ranges of the present invention. Unexpectedly, such compositions at critical average hydride equivalent weight (HEW) levels can be obtained at relatively low viscosity from proper design and selection of the hydride-terminated fluid and the hydride crosslinker fluid.